The invention relates generally to the field of solar power, and more specifically to methods of detecting arc faults in photovoltaic solar power arrays.
The solar power industry is facing a growing safety concern over arc faults. The reason relates to the way in which conventional photovoltaic solar power arrays are constructed, producing high voltage dc in a circuit with a large number of series connections. For example, a typical photovoltaic cell produces about 0.5Vdc in full sunlight, and a typical solar power module includes seventy two cells. Therefore, an array of sixteen such modules wired in series would produce about 576Vdc, and the system would have 1152 series connections (not counting additional connections in the junction boxes, combiner boxes, cable connectors, inverter, etc.). If any one of these connections opens, then the entire output voltage of the solar array is concentrated across the small gap, resulting in an arc that can reach temperatures of several thousand degrees, and the arc is often difficult to extinguish.
The industry has been building conventional arrays like this for a long time, and there are now a large number that have been in the field for twenty years or more. These systems are beginning to show signs of age, and the annual number of reported fires is starting to rise. This is why the 2011 National Electrical Code (NEC) now requires an Arc Fault Circuit Interrupter (AFCI) in solar arrays that produce more than 60Vdc.
Various companies are working on developing AFCI products for solar power arrays. FIG. 1 shows a typical example of a solar power system 1 that utilizes conventional AFCI. An array 2 of conventional solar power modules 3 and blocking diodes 4 is connected to an inverter 5 via a power bus with positive 6 and negative 7 rails. A conventional arc detector 8 is coupled to the power rails via capacitors 9. When an arc is detected, the arc detector 8 opens the power switches 10, thereby interrupting the current, and thus extinguishing the arc.
The conventional arc detector 8 looks for the spectral signature generated by the arc. FIG. 2 shows a typical example of such a signature, where the current amplitude is approximately inversely related to the frequency; a relationship that is commonly referred to as a 1/f spectrum. Since both axis of the graph in FIG. 2 are logarithmic, a 1/f spectrum appears as a straight line with a downward slope as shown.
The conventional solar power system 1, and the like, face several technical challenges. First, a large solar array can act as a large antenna, picking up noise from the environment and creating the potential for false alarms. Second, the array 2 can act as a filter that distorts the arc signature, or makes environmental noise look like an arc signature. And third, the arc detector 8 can detect the presence of the arc, but cannot locate it; so it may be difficult to get the system 1 up again after a shutdown.
The problem of not being able to locate the arc is significant. If the system 1 works properly, it shuts down before any significant damage occurs, so there may be little visible evidence of the arc. In fact, arcs can occur inside the junction box on the back side of a solar module, where they cannot be seen without disassembly of the module. What is more, a typical solar power array can cover a large area. Searching for a small burn pattern in a large array can be like looking for the proverbial needle in a haystack. An operator may be forced to power up the system repeatedly until enough damage accumulates to make the fault location apparent. And what is worse, if the arc cannot be found, then maintenance personnel may mistakenly assume the shutdown was a false alarm. This is worrisome because some people may actually try to circumvent the AFCI system if the “false” alarms persist.
Accordingly, there is a strong need in the field of photovoltaic solar power for a method of reliably detecting an arc fault, and determining its general location. The present invention fulfills these needs and provides other related benefits.